The present invention relates to a pulley actuating hydraulic pressure control system for a continuously variable transmission, and more particularly to a line pressure control system for a continuously variable transmission.
Laying-open Japanese Patent Application No. 60-30854 discloses a line pressure control system for a stepped automatic transmission. According to this known line presure control system, a line pressure is increased temporarily when a manual valve is placed from a D (drive) range position to a L (low) range position. With this increased line pressure, a hydraulic force large enough for effecting rapid shifting toward a large reduction ratio is generated.
EP No. 0180209 or copending U.S. patent application Ser. No. 922,400, filed Oct. 23, 1986, now U.S. Pat. No. 4,735,113, discloses a continuously variable transmission. In this known continuously variable transmission, a V-belt between a driver pulley and a follower pulley tend to slip during a transient period upshifting from a reduction ratio state to a smaller reduction ratio state. Such a slip is noticeable when a manual valve is selected from a L range position to a D range position during acceleration of the vehicle because with the same vehicle speed and throttle opening degree, a smaller target reduction ratio is given for the D range than a target reduction ratio given for the L range, and the driver pulley expands quickly to increase the width of its V-belt receiving groove. During this quick upshifting operation, the inertia of the pulleys cannot keep a tension on the V-belt to cause a slack to occur. This results in a shortage in torque transmission capacity of the V-belt, and causes the V-belt to slip immediately after the manual valve has been moved from the L range position to the D range position. Even though the slip of the V-belt occuring during the above-mentioned transient period may be reduced if the line pressure is increased to a sufficiently high level, the V-belt will wear at an increased rate if the line pressure increased to the sufficiently high level is always supplied to the pulleys, This shortens the operating life of the V-belt. Besides, this causes an increased operating loss of the associated oil pump.
The present invention aims at providing a hydraulic pressure control system for a continuously variable transmission which can prevent a V-belt from slipping during the above-mentioned transient period without shortening the operating life of the V-belt.